1. Field of the Invention
The invention relates to sonar transducers for use in sonar arrays, and more particularly to a dual frequency sonar transducer assembly which may be operated at low and/or high frequencies in a sonar array.
2. Prior Art
A sonar transducer is a device for generating sound and sensing sound in water. A sonar transducer is at heart a resonator which in the case of ceramic sonar transducers, includes an electroded ferroelectric member. The application of electrical potentials to the electrodes excites mechanical motion in the ferroelectric member used to generate sound waves in the water, and mechanical forces exerted upon the ferroelectric member by sound waves in the water is used to generate an electrical potential in the electrodes to sense the sound.
A common form of sonar transducer includes a "stack" of ring shaped drivers, electrically connected in parallel, clamped by means of a stress rod between a tail mass, which is relatively heavy, and a head mass, which constitutes a relatively light, water driving piston. The tail mass, ceramic stack, and head mass form a two mass resonator assembly. The arrangement desirably produces small amplitude vibrations in the tail mass and large amplitude vibrations of the head mass which acts as a water driving piston.
A transducer is referred to as a "tonpilz" design when the resonator at its heart has the lumped elements described above. The tonpilz resonator may be distinguished from quarter wave and half wave resonators in its use of lumped elements as opposed to distributed elements. In mechanics, the elements which define the resonant properties of an ideal resonator are masses, springs, and sources of loss. Neglecting losses, the tonpilz resonator may be regarded as having a central spring--the driver resilience, and two masses--the head mass and tail mass. The half wave resonator, a practical example of a distributed design in a sonar transducer, consists of a simple monolithic member of ferroelectric material in which the mass and resilient (spring) properties are distributed through the member.
The half wave resonator with its distributed design is often less desirable than a lumped element tonpilz design in which the properties of the lumped elements may be individually optimized. For instance, by adding a dense head mass and tail mass to a ferroelectric driver of conventional density and compliance in a longitudinal mode tonpilz design, one may achieve a shorter length than can be achieved in a half wave resonator. In the half wave resonator operating at the same frequency, the same ferroelectric material is used to provide both the distributed mass and the distributed resilience. With the densities of available ferroelectric materials being less than those of metals usable for masses, the half wave longitudinal resonator is necessarily longer than the tonpilz resonator.
The tonpilz transducer is a relatively narrow band, resonantly operated, single frequency device. It is often desirable to have additional operating frequencies beyond a single fundamental frequency, which is generally all that is available. The advantage of a multiple frequency transducer, if compatible with assembly into a sonar array,is greater versatility. Since a lower frequency may provide greater detection range, and a higher frequency greater spatial resolution, a transducer which operates on two appropriately selected frequencies is of substantial value and requires no additional aperture area than would be required for an array operating on a single frequency.